Over the last two decades, emphasis has been placed on the importance of regularly exercising the human body. Medical science has established that enormous benefits can be achieved from regular exercise, such as reduction of in an individual's cholesterol level, reduction of overall body fat and stronger, healthier heart and lungs. In fact, it has been shown that one of the best forms of exercise involves one that provides the body with a thorough "aerobic" workout by providing an adequate amount of resistance that allows the user to sustain a rapid heart rate for an extended period of time of between 20 to 40 minutes.
Various types of exercise equipment designed to provide the user with such aerobic resistance machines have been developed and are well known in the art. For instance, one type of exercise equipment employed by individuals to aerobically exercise the body is free weights or "bar bells." While this can provide the user with an excellent form of variable weight resistance and aerobic activity, free weights suffer from several disadvantages. For instance, free weights are both heavy and bulky. Moreover, to perform the different types of exercise presently recommended by physical fitness experts, the user must be able to use the weights in diverse lifting motions, many of which cannot be performed using free weights. Additionally, the user must either have several weight amounts set up at any given time so that the user can move quickly from one exercise to another, or the user must stop each time and change the weights out for the next exercise--a stop that greatly inhibits the aerobic benefits of the workout. To overcome these disadvantages, rack weight machines have been provided that are assembled to allow the user to easily move from one weight station to another and quickly set the weight amounts, thereby providing a machine that is capable of delivering a resistance/aerobic workout. However, these machines are also often bulky, heavy and take up a significant amount of space.
Over the years, exercise machines have undergone vast changes in an attempt to provide the user with an exercise machine that is capable of providing the user with a compact, light-weight form of resistance and simultaneously give the user a good aerobic workout. These devices typically employ diverse forms of mechanical apparatus to achieve these goals, such as bicycling, rowing, jogging, striding and stair climbing apparatus. For example, U.S. Pat. Nos. 4,645,200 to Hix; No. 4,989,858 to Young, et al.; No. 5,104,363 to Shi; No. 4,850,585 to Dalebout; and No. 4,940,233 to Bull, et al. generally illustrate these types of apparatus. The patent to Dalebout U.S. Pat. No. 4,850,585 discloses a striding-type apparatus. The Dalebout apparatus includes a frame and a pair of reciprocating leg members that support a user above a supporting surface, such as a floor. The user stands on foot supports connected to the leg members and moves his legs in a striding-type reciprocating motion. A pair of handle members may also be associated with the leg members to rotate simultaneously therewith. A reciprocation mechanism may be provided to force opposite rotation of the leg and arm members with respect to each other. However, such striding-type apparatus have distinct disadvantages. First, they typically provide resistance in only one direction, particularly with respect to the upper body. Second, they do not closely simulate the natural striding movements of both the arms and legs during walking, as the feet are forced to travel in a straight line, rather than in an arc about the user's hip joint.
Some of the more recently developed machines use a resistance that is created by a complex system of electronically actuated brake-type devices that are controlled by microprocessors. Although these systems can provide the user with a resistance and aerobic workout, they suffer from the disadvantage that they are susceptible to mechanical failure and regular maintenance requirements due to their complex integrated mechanical and electronic design.
Many of the other apparatus previously mentioned, which provide the user with resistance and aerobic forms of exercise, incorporate the use of hydraulic of resistance cylinders. Generally, these hydraulic cylinders use incompressible hydraulic fluids to form the resistance against which the user exerts himself. While, these devices are typically compact and light weight, they too have disadvantages associated with their use. For example, they typically provide the user with resistance in only one direction. Thus, the user cannot achieve a maximum resistance and aerobic benefit during the time in which he is using the machine.
Another disadvantage lies within the hydraulic cylinders that are typically employed in such devices. The hydraulic cylinders generally used are conventional shock absorbing-type cylinders. For example, a typical structure of such a device includes a cylinder having a piston disposed therein, the piston having a fluid port therethrough and attached to a rod extending externally from the cylinder that is mechanically attached to a lever arm. The cylinder has an incompressible hydraulic fluid or oil within the cylinder that is forced from one end of the cylinder to the other via the fluid port in the piston as the piston is reciprocated within the cylinder. During use, the hydraulic fluid's temperature increases due to the friction that is created from the fluid being forced back and forth through the piston's fluid port. Since the fluid is confined to a rather small volume with little surface area, the heat is not easily dissipated from the fluid. Thus, the temperature of the fluid quickly increases and, since hydraulic fluids have a positive viscosity index, the heat quickly breaks down the fluid's viscosity. The resistance of the cylinder is thus decreased because the fluid moves more easily through the fluid ports in the piston, and the resistance and aerobic benefits to the user are decreased, as well. Moreover, the lower viscosity may also cause the fluid to leak from the seals in the cylinder.
Another disadvantage of the conventional exercise hydraulic systems is that the fluid ports cannot easily be adjusted to change the resistance of the hydraulic system. Typically, to change the resistance, the user must interrupt his exercise routine and manually adjust the mechanical advantage of the lever arm with respect to the cylinder to either increase or decrease the resistance. This interrupts the user's aerobic activity and therefore decreases the effectiveness of the exercise routine.
Therefore, it can readily be seen that there is a need in the art for a hydraulic system for an exercise apparatus that uses a compressible fluid to absorb energy. There is also a need in the art for a fluid having a more stable viscosity index, such that viscosity is maintained at elevated operating temperatures. Further, there is a need in the art for exercise equipment that is compact, relatively light weight and durable and provides an aerobic routine with user adjustable, bidirectional resistance.